Modeling The Rollovers of Sandy Clinoforms from the Gravity Effect On Wave-Agitated Sand

Clinoforms on coasts exposed to ocean waves form from sand exported from the shoreface during extreme wave conditions and deposited where wave action diminishes. Elsewhere, it is shown that during upper 5-percentile wave conditions, wave-induced shear stresses exceed the sediment threshold of motion to below the clinoform rollovers. Experiments and theory suggest that, where bed sediments are agitated by waves, the effect of gravity should move particles down-slope with a flux proportional to the slope. Combined with considerations of continuity, this implies a diffusion of the sediment topography, a property that would explain the smooth seabed morphology found at sandy rollovers where recorded with multibeam sonar. For situations where this gravity effect dominates, a simple analytical expression developed here shows how the rollover curvature should relate to wave properties and to the offshore component of sediment flux. More sharply curved rollovers are expected where waves have short periods or where the sediment flux is large. Relative sediment fluxes were calculated using the model from rollover curvature and wave properties for sites from California, southeast Australia, and Atlantic and Mediterranean Iberia. The relative magnitudes of the fluxes are roughly as would be expected from local physiography and coastal erosion rates. For sandy clinoforms developed under mainly wave influences, the model could be useful for exploring variations within clinoform datasets (e.g., how varied convexity in an area of uniform wave properties reflects varied sediment flux) and for interpreting how varied curvatures of rollovers within seismic stratigraphy reflect how wave climate and flux have varied in the past.